The Truth About Canon Speedlites & Optical Slaves

So, how do you get a bunch of Canon Speedlites to fire…

at the same time that you fire a bunch of other types of flash? Say you have a Quantum Qflash, a Metz 58 AF-1, a LumoPro LP160, and a Calumet Travelite. Then say that you need to take a group portrait of these guys as they are firing and light it with Speedlites. How do you get everyone to fire at once? You use optical slaves…a whole bunch of them. [If you’ve heard the myth that Canon Speedlites don’t work with optical slaves, mark that person down on your holiday card list as a misinformed Speedliter.]

An optical slave is an electronic eye that will trigger a Speedlite when it sees a large burst of light. Since an optical slave literally works at the speed of light, it will fire an off-camera flash at essentially the same time that your on-camera Speedlite fires. Many brands of flash, other than Canon, have optical slaves built in. Since ours do not, we have to add them externally.

The upside of optical slaves:
+ Relatively inexpensive
+ Eliminates cords from running through the frame
+ Enables Speedlite to be concealed within the scene
+ No batteries required
+ Can mix Speedlites and other flashes

The downside of optical slaves:
– Most optical slaves are not Canon-compatible
– Range limited to about 30’/9m
– Must have clear line-of-sight between on-camera flash/trigger and Slave
– Can be fired by other nearby cameras
– Speedlites must be fired in Manual mode, unless you have a $pecial digital slave

Optical Slaves: The Basics

The triggering burst for an optical slave can come from any flash or an infrared trigger mounted to your camera. So, think of the optical slave as a receiver and the flash or infrared trigger as the transmitter.

An optical slave is an inexpensive way to trigger an off-camera Speedlite when you want to avoid the hassle of cords and connections. Keep in mind that the connection is line-of-sight—meaning that you cannot fire optical slaves hidden on the other side of a wall. For that, you have to use a radio trigger. Also, optical slaves won’t set the power of your Speedlite. You’ll have to do that yourself.

Optical Slaves & E-TTL Don’t Mix

There are two other caveats to using optical slaves with Canon Speedlites. The first is that, unless you use a $pecial digital slave, you’ll have to operate your Canon Speedlites in Manual mode. This is because optical slaves don’t think. They just connect the circuit and trigger the flash when a burst of light comes along. When you are shooting E-TTL or with red-eye reduction activated, there is a pre-flash before the actual exposure. The pre-flash causes the optical slave to fire the Speedlite prematurely. All Canon pop-up flashes always emit a pre-flash—except the 7D when in Manual flash mode. So, to get rid of the pre-flash, switch your Speedlite(s) from E-TTL to Manual mode — and don’t use a pop-up flash.

Canon Speedlites Need A Special Optical Slave

The other issue (and this is the reason for the myth) has to do with the working voltage of Canon Speedlites. An optical slave gets its power from the Speedlite rather than from a battery. With Canon Speedlites there is the peculiarity that the voltage does not drop far enough after the flash exposure to release the typical slave circuit. Essentially, the slave thinks that the Speedlite is still firing. With ordinary optical slaves, the result is that they will fire a Speedlite one time and then lock up. Fortunately, there is an optical slave with an added circuit that is made just for Canon Speedlites. And, luckily, this technology adds less than the price of a venti cup of fancy coffee to the cost of the optical slave (about $16 as opposed to $12).

Sonia’s Canon EX-Compatible Optical Slaves

^^^ Sonia makes optical slaves with orange, yellow, and green bases. Only the green-based slaves will work with Canon Speedlites.

Sonia is a leading manufacturer of optical slaves. As Canonistas, the key for us is to buy the green ones. The standard yellow and orange versions are not for us. The secret to the green slaves is that they have the additional circuit that makes them compatible with the power cycle of Canon Speedlites.
Sonia offers Canon EX-compatible slaves in both mono miniphone and PC-male. You can buy Canon-compatible Sonia slaves here on FlashZebra.com.

^^^ The male PC-jack will plug directly into a 580EX II. For other Speedlites, I prefer a miniphone jack as it provides a more reliable connection into the hotshoe adapter.

Connecting Optical Slaves To Canon Speedlites

580EX II: An optical slave with a PC-male jack can be plugged directly into the PC socket on a 580EX II. This direct connection is the only reason I have slaves with the sometimes-it-works-and-sometimes-it-doesn’t, old-school PC-sync connection. The key to obtaining PC-sync happiness is to carry a little wrench-thingie (a “PC tip conditioner“) that you can use to periodically tighten the flanges on the PC-male fitting.

All other Speedlites: You will need a hotshoe adapter for the flash into which you plug the optical slave. Be sure to get an adapter that has a threaded socket in the bottom so that you can connect it securely to your lightstand (these are called “flash hotshoe adapters” as opposed to “camera hotshoe adapters”). I prefer adapters that have a miniphone socket over those that have just a PC-sync port. The miniphone is a much more reliable connection than the ancient PC-sync. Flash Zebra has a good selection of miniphone hotshoe adapters here.

Infrared Triggers

An infrared trigger is an alternative to firing optical slaves with an on-camera flash. Infrared waves are just below that part of the electromagnetic spectrum that we see as red. While we can’t see infrared, optical slaves can. Infrared triggers work much like a manual flash in your camera’s hotshoe—except that the flash is not seen—which is great if you want to avoid the pitfalls of on-camera flash.

The two downsides to infrared are that infrared is not reliable outdoors on bright / hot days and that infrared has a limited range—approximately 15 to 30 feet. At least the infrared triggers themselves are relatively inexpensive—when compared to the cost of radio triggers.

Flashpoint and Wein are two manufacturers of infrared triggers. The Wein (about $60 here) is roughly twice the price of the Flashpoint and offers more robust construction. If you are just getting started, the Flashpoint (about $30 here) will get the job done nicely. Both activate optical slaves with reliability.

Ryan – Good question. No, a simple IR trigger will not work to control Canon Speedlites as slaves via the built-in system. An IR trigger is a simple device that sends one message — "fire now!" It's a single pulse. The Canon wireless system uses a complicated series of pulses in the E-TTL pre-flash to communicate instructions between the master and slaves. Even when shooting the slaves in manual mode wirelessly, there is still the long string of pulses…think of them as Speedlite Morse code.

Thank you! This might (fingers crossed) be the answer I've been looking for. I have one flash off-camera on pocket wizards. But I can't get it to trigger a second flash using Canon's wireless controls (I assumed the camera would think the first flash was still on camera when the PWs were installed). So if I read this article correctly, I can plug in the optical adapter to the second flash and viola!

Harry – using Canon’s built-in wireless AND PocketWizards requires the use of a PW MiniT1 or FlexT5 on-camera. So, with the gear you have, switch your Speedlites into Manual and use the green Sonia on your second off-camera Speedlite. Just remember, the slave has to be able to see the flash from the triggering Speedlite.

[…] credit for this. I learned about it on Syl Arena’s blog, Speedliting. His blog post, “The Truth About Canon Speedlites & Optical Slaves” opened my eyes to something I didn’t think was possible with Canon Speedlites and I […]

Can I use a 580EX as slave only without a physical connection to the camera or as a master on the camera?,(5d Mark III) I have Elinchrom Studio lights and they are working with a wireless trigger and also as a slave if I connect the 580EX on the camera, but i cannot get my speedlight to fire from anything but the hot-shoe…if the 580ex fire, yes the studio lights respond, but not the other way around!!! Do I need a wireless adapter on the 580EX? It does not respond on my wireless adaptor of my Elinchrom lights.

Syl — I learned lighting reading your book and I think it was a fantastic knowledge base from which to start, so thanks for that! I’m now trying to mix my Speedlites with studio strobes using optical slaves and am having an issue that I do not think I’ve heard you (or anyone else) address before: the slaved Speedlites fall asleep quickly and stop firing. I have to keep a constant eye on them and “wake them up” as necessary. Is there a setting that will keep them on continuously when slaved?

This was extremely helpful. I just want to check something with you before I purchase one. WOuld it be possible for this to still work if I had a pc cync chord inbetween the two? I ask this as I am putting it in a housing of some sort and there isn’t much room to the side of it.

Syl,
After watching your training video on getting the most out of Speedlites, i decided to get my lighting game on point. I already had two 430exiis. I bought 2 Lastolite Ezybox 24x24s and a 600exii in hopes that that the 430s would slave off the master 600. The 430s worked great until I put them in the Ezybox,then they stopped firing. I suspect that the problem is that the support arm for the Ezybox is blocking the infrared line of sight. Is there any way to get this setup to work or would I be better off selling the 430s and getting another 600?

@Patrice – The optical eye on Canon Speedlites is just above the red panel on the front of the flash. Ideally, the front of the slave Speedlite faces the master Speedlite (using the pan & tilt to angle the head at the subject). With the Ezyboxes, twist the body of the slave 90-degrees to the right or left so that the slave sensor is not blocked by the softbox. Selling the 430s and getting another 600EX-RT is an option if you have a way to trigger them. To fire them as radio slaves, you will either need: an ST-E3-RT transmitter or a 600EX-RT in the hotshoe or an extra-long ETTL cord that will enable you to connect one of the 600s to the hotshoe while mounting it on the Ezybox. That corded Speedlite would then talk by radio to the other 600 (set-up as a radio slave). I have a little biz that sells those cords — OCF Gear.

I want to add that the Canon flash system is a most ridiculous system. I have an entry level Nikon and Canon camera and firing an optical slave from my Nikon camera is a dream, the Canon is an impossible nightmare. I recognized that this is an old article but with a bit of an update is still very relevant with one addition, Canon speedlites are useless with optical triggers. The reason is very simple, there is no way to disable TTL, even in manual mode. Holy Crap! I always thought that manual was the opposite of TTL, I just realized I was wrong. I want to fire my flash at 1/32 power, that’s it, nothing more. What is the pre-flash sending out?

If, by “optical slave” you mean setting your Speedlight to SU-4 mode so that it fires when it sees a flash of light from any camera, then yes this is a feature that is not built into the Canon system. However, in Nikon’s SU-4 Mode, you must adjust the power directly on the Speedlight each time you want to change it.

If, by “optical slave” you mean than the pop-up flash on the camera or the Speedlite in the hotshoe controls the off-camera slave via a coded series of pre-flashes, then the Canon system has the advantage of enabling the photographer to change Speedlite settings from the LCD of the camera. Among the settings that can be controlled from the camera’s LCD are Mode (ETTL or Manual) and, if in Manual mode, the specific setting at which the slave will fire. For me, it is much more convenient to do this on my camera than to walk over to the Speedlite each time I want to make a change in settings.

The wein peanut works with preflash and won’t lock up. I’ve still been setting my slaves to manual (Usually using old non canons, haven’t tested speedlites yet) but I can leave my main flash on ETTL no problem